1. Field of the Invention
This invention relates to a magnetic disc for magnetic recording in a direction parallel to or perpendicular to its surface.
2. Background Art
One example of a conventional magnetic disc is a rigid magnetic disc. The substrate of the rigid magnetic disc is made of rigid material. In general, the rigid magnetic disc employs an aluminum substrate JIS H 4000 A5086P for instance).
The rigid magnetic disc is generally formed as follows. An aluminum plate is machined on a lathe, and its surface is polished so that a data recording operation can be performed at high density because the space between the disc and the head is minimized. Thereafter, a magnetic layer is formed on the aluminum substrate by vacuum deposition or spin coating. For high density recording and reproducing, the surface of the disc should be smooth. However, it has been difficult for the conventional magnetic disc using the aluminum substrate to have a surface with a central line average roughness Ra of not more than 0.1 micrometers. Furthermore, since the substrate is rigid, in forming the magnetic layer, it is impossible to continuously coat the web with a magnetic material. Thus, handling of the conventional rigid substrate is rather troublesome. On the other hand, a high density recording operation is greatly affected by the dust on the disc. Therefore, it is essential to manufacture magnetic discs in a dust-proof environment. Accordingly, the magnetic disc manufacturing process is considerably intricate, thus requiring an enormous sum for investment in equipment.
The conventional substrate, such as an aluminum substrate, is rigid. Therefore the head should not contact the magnetic layer in tracing it. However, it is difficult to maintain constant the narrow space between the head and the magnetic layer, and therefore signal errors frequently occur. Further, it is very difficult to further reduce the space between the head and the disc surface to thereby increase the recording density. If the magnetic head is accidentally brought into contact with the disc during the tracing operation with the narrow space maintained, a sliding frictional force is produced because the substrate is rigid. As a result of this frictional engagement, a great impact is applied collectively to the part of the magnetic layer which has contacted the head, so much so as to break the disc surface, thus reducing the service life of the disc.
Furthermore, the aluminum substrate polished as described above is expensive.
On the other hand, a magnetic disc as shown in FIG. 1 has been proposed in the art. In this magnetic disc, recesses are formed in both sides of a disc substrate 1, and floppy discs or flexible discs 2 (hereinafter referred to as "flexible disc sheets") having a magnetic layer on one side are bonded on both sides of the substrate 1 in such a manner that the magnetic layers are exposed on the exterior of the disc and gaps 3 are formed between the substrate 1 and the inner surface of the flexible disc sheets 2.
The magnetic recording surfaces of the magnetic disc thus constructed are flexible. Therefore, even when the head is accidentally brought into contact with the magnetic recording surface, or when a higher density recording operation is carried out with the head held in contact with the magnetic recording layer, the magnetic layer is scarcely broken, unlike that of the rigid disc. Accordingly, the technique of flexible discs can be utilized as it is. Furthermore, a smooth magnetic layer that is highly durable can be used as a magnetic disc's magnetic layer. Thus, this type of magnetic disc is being watched with great interest since it eliminates the difficulties accompanying a conventional rigid magnetic disc.
The inventors have conducted intensive research on the magnetic disc of this type, and found that it still suffers from serious difficulties. In particular, when the flexible sheets are merely bonded on both the interior and exterior edges of the substrate 1 in such a manner as to form the gaps 3 as shown in FIG. 1, the magnetic disc may eventually be bent by the force of contraction of the flexible sheets 2 or when held at a high temperature (about 70.degree. C.). Furthermore, the magnetic disc is bent by the variation of temperature or humidity. That is, the magnetic disc may be bent with variations of the environmental conditions.
The bending of the magnetic disc because of aging or high temperature is not reversible because it plastically deforms the substrate 1. On the other hand, bending of the magnetic disc because of temperature or humidity change is reversible and is generally too small to practically affect the magnetic disc. In the case where the magnetic disc is irreversibly bent by the contraction as was described above, spacing of the magnetic head from the disc surface becomes unsatisfactory, so that the output is decreased, at worst to zero. In fact, what occurs is that the spacing is reduced to zero as the head hits the disc surface, that is, the aimed high density recording operation cannot be carried out.